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Discovery Methods

IC 1459

Determining the mass of a black hole in the center of a distant galaxy is never easy. And in the case of IC 1459, a galaxy that's about 100 million light-years away, it's proved frustratingly difficult.

In 2002, a team of astronomers reported results of a study using instruments on the ground and aboard the Hubble Space Telescope. The team's goal was to measure the motions of both stars and clouds of gas at the galaxy's center. Stars and gas orbiting close to the black hole would move faster than those at greater distances. Measuring the orbital speeds at different points in the galaxy, and comparing them to the total amount of material in the galaxy, should reveal the black hole's mass.

IC 1459 is an elliptical galaxy, so it is shaped like a fat, fuzzy football. As with all elliptical galaxies, stars in the galaxy's outer regions move in random orbits around the galactic center. A disk of gas and dust dominates the galaxy's center, with a small disk of stars at the center rotating in the opposite direction.

Determining the geometry of the wide dust disk and the smaller core are key factors in determining the mass of the black hole. Astronomers use models that are sensitive to such factors as the angle at which we view the disk, the disk's thickness, whether it is flat or warped, and whether it is symmetrical or off-center, among others.

In the case of IC 1459, however, the astronomers were unable to accurately determine all of these factors. As a result, their work yielded a wide range of possible masses for the central black hole. The motions of the gas yielded masses of as little as 350 million times the mass of the Sun to as much as 1.1 billion times the Sun's mass. The motions of stars, however, produced a measurement of 2.6 billion times the Sun's mass (with an uncertainty of more than one billion solar masses).

An earlier study, also using Hubble and ground-based observations, came up with a smaller mass for the black hole. That study used a closer estimate for the galaxy's distance, however. When the distance is scaled to that of the 2002 study, the range was about 200 million to 600 million times the Sun's mass.

Regardless of the numbers, however, all the models of both study groups showed that the galaxy contains a central supermassive black hole. The galaxy's core also produces radio waves, which are another signature of a supermassive black hole, and there is a small, blue object at the galaxy's center that could be either an accretion disk or a cluster of hot young stars around the black hole.

Astronomers will have to keep looking, though, to come up with a more precise measurement of the black hole's mass.

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